Proton exchange membrane fuel cells (PEMFC) are considered to be one of the most promising fuel cell technologies for vehicular machines, unmanned aerial vehicles (UAV), portable power applications and stationary power applications. The use of hydrogen as the feedstock for PEMFC systems is thought to be an environmentally friendly and a clean source of energy. Since hydrogen fuel reacts with oxygen to produce electrical energy and water only and as a result has the potential to solve many environmental problems associated with conventional petroleum based fuels.
At present, high pressure vessels are commonly used for hydrogen fuel storage. However, these high pressure vessels are not ideal for highly portable applications, such as the UAV, owing to a combination of the low hydrogen density (HD) as well as the excessive weight of the high pressure vessels. Due to such problems there is an increasing demand for alternative hydrogen fuel technologies for use in highly portable applications.
Dated back to World War II, while working with Hermann Irving Schlesinger, Nobel Laureate Herbert C. Brown discovered a method for producing sodium borohydride. This discovery led to the development of metal borohydrides as viable hydrogen carriers (U.S. Pat. Nos. 2,461,662, 2,461,663, 2,534,553 and 2,964,378). During the 1990s, sodium borohydride attracted a tremendous amount of attention due to its chemical properties including non-flammability of sodium borohydride solutions, high hydrogen density (HD, 10.8 wt. %) and the high stability of its environmentally safe reaction by-products.
U.S. Pat. No. 6,534,033 describes a hydrogen generation system wherein the hydrolysis of sodium borohydride has been successfully demonstrated. However, hydrogen generation using this system does not appear to be suitable for heavy-duty applications due to issues relating to the handling of water, catalyst reactivity/deactivation and the treatment of by-products. These issues have been further discussed in the following article (J. H. Wee, K. Y. Lee, and S. H. Kim, Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems, Fuel Processing Technology, 87 (2006) 811-819).
It is know that the hydrolysis of sodium borohydride to produce hydrogen may be accelerated with the aid of catalytic materials packed into a reactor. Accordingly, the development of heterogeneous catalysts for the hydrolysis of sodium borohydride to produce hydrogen has become an area of extensive research. In 1962, Brown et al. reported the use of the noble metal ruthenium as a catalyst for accelerating the hydrolysis of sodium borohydride wherein a hydrogen generation rate of 85.4 L min−1 g−1 was reached (H. C. Brown and C. A. Brown, New, Highly Active Metal Catalysts for the Hydrolysis of Borohydride, Journal of the American Chemical Society, 84 (1962) 1493-1494). To-date, ruthenium-based catalysts display the highest catalytic effect on the hydrolysis of sodium borohydride. However, the price of ruthenium is relatively high (RuCl3.nH2O costs approximately 8500 SGD/kg). A recent metal price comparison (June 2011) showed that ruthenium costs approximately 180 USD/oz, while cobalt costs only approximately 16 USD/lb, or a price ratio of ruthenium to cobalt of 180.
In 1953, Schlesinger et al. reported the catalytic effect of manganese, iron, cobalt, nickel and copper chlorides on the hydrolysis of sodium borohydride to generate hydrogen. They disclosed that the catalytic effect of cobalt chloride was higher than that of other metal chlorides, and concluded that the effect of the cobalt salt was ascribed to the catalytic action of cobalt boride (Co2B), which was formed in the initial stages of the reaction (H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R. Hoekstra, and E. K. Hyde, New Developments In The Chemistry Of Diborane And Of The Borohydrides .9. Sodium Borohydride, Its Hydrolysis And Its Use As A Reducing Agent And In The Generation Of Hydrogen, Journal of the American Chemical Society, 75 (1953) 215-219).
It appears that the use of cobalt-based catalysts, for sodium borohydride hydrolysis, may be attractive alternatives to ruthenium based catalysts, owing to their high reactivity and cost effectiveness (B. H. Liu and Z. P. Li, A review: Hydrogen generation from borohydride hydrolysis reaction, Journal of Power Sources, 187 (2009) 527-534). Various forms of cobalt have been extensively researched in the literature, including chloride-based, metallic-based, boride-based, alloy-based (of boron (B) or nickel (Ni)), phosphorus-doped, carbon-supported, resin-supported, metal oxide-based and thin film-based forms. However disadvantages associated with the price of such nano-size cobalt-based catalysts and the fact that the “spent” catalyst cannot be easily separated or recycled has caused concern for those using such catalysts. Furthermore, the durability of such catalysts has raised concerns, especially when used for multiple harsh start-stop applications (S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo, and M. Binder, A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst, International Journal of Hydrogen Energy, 25 (2000) 969-975).
Porous/dense metal oxide beads have also received growing attention in recent years for their potential use as catalyst, catalyst supports, diesel exhaust filters, pen balls, and grinding media. The specific use of the metal oxide beads is often dependent on factors such as the bead dimensions, degree of porosity, nature of the porosity and the metal oxide composition. Fabrication of porous/dense metal oxide beads has been widely demonstrated. For example, U.S. Pat. No. 3,331,898 discloses a method whereby an aqueous sol stream of said compounds is formed into sol droplets of 200˜500 μm in size. This is performed by passing the sol stream into an organic drying liquid stream (at an angle to its direction of flow) thus producing a high shearing force on the said sol leading to droplet formation.
U.S. Pat. No. 6,197,073 discloses a method of producing aluminum oxide-based beads. During this method, an acid aluminum oxide sol or an acid aluminum oxide suspension is converted into droplets using a vibrating nozzle plate. Pre-solidification then occurs, after formation of a bead shape, by laterally blowing over with gaseous ammonia before coagulating in an ammonium solution.
U.S. Pat. No. 4,106,947 discloses a method of fabricating zirconium oxide-based beads by fusion. U.S. Pat. No. 5,502,012 then improved the quality of the zirconium oxide-based beads, fabricated via fusion, by optimizing the composition of raw materials used.
U.S. Pat. No. 4,621,936 discloses the reaction of soluble alginates with a water solution of multivalent metallic salts to form gels. For example, zirconia was added to a solution containing ammonium alginate to form homogeneous slurry. The homogeneous slurry was then dropped into a calcium chloride solution to form spherical beads via a reaction between the ammonium alginate and the alkaline halide. U.S. Pat. No. 6,797,203 also discloses a similar system for fabricating ceramic beads. U.S. Pat. No. 5,322,821 discloses a method for preparing porous ceramic beads from aluminium oxide (Al2O3) powders by mixing the powders with a surfactant and sodium borate. The mixture is then poured into hydrocarbon oil and emulsified using a high shear agitator at approximately 300 rpm leading to the formation of foam spheres of finer and more uniform size. U.S. Pat. No. 7,517,489 discloses a method for preparing ceramic, metal and mineral beads with a diameter in the range of 0.1-10 mm using one kind of grain flour as binder. Additional methods which employ gel-casting technique are disclosed by Guo et al., (CN 1935478A), Chen et al., (CN 101066884A), and Yang et al., (CN 1241876C). However, these methods include disadvantages since the system set-up can be arduous because the length of hot oil tube needed is approximately 1˜10 m in length.
The forming of metal oxides with complex shapes can be a time consuming, expensive and in many cases impractical process. Gel-casting has been demonstrated to be a unique process for forming complex or intricately shaped parts from metal oxide powders. In this process, slurry with a high solid state loading is obtained by dispersing the metal oxide powders in a pre-mixed monomers and cross-linking solution. With heating or addition of a catalyst, cross-linking polymerization occurs to form a three-dimensional network structure, and the slurry is solidified in situ, to form solid objects of the desired shape. The resulting green product is of exceptionally high strength. After drying, the green product can also be further heated to remove the polymer and can also be fired or sintered to produce a metal oxide product. Gel-casting methods have been studied in Janney (U.S. Pat. Nos. 4,894,194, 5,028,362 and 5,145,908) and Claudia (U.S. Pat. No. 6,066,279). During these studies a great amount of attention is focused towards the improving of the gel casting technique, however little effort was paid towards the shape of the product formed.
Gel-casting techniques have been used to synthesize powders for solid oxide fuel cells. This was reported by Zhang et al., where pervoskite powders were synthesized using gel-casting techniques. (L. Zhang, Y. J. Zhang, Y. D. Zhen and S. P. Jiang, Lanthanum Strontium Manganite Powders Synthesized by Gel-Casting for Solid Oxide Fuel Cell Cathode Materials, Journal of American Ceramic Society, 90(5) (2007) 1406-1411. L. Zhang, S. P. Jiang, C. S. Cheng and Y. J. Zhang, Synthesis and Performance of (La0.75Sr0.25)1-x(Cr0.5Mn0.5)O3 Cathode Powders of Solid Oxide Fuel Cells by Gel-Casting Technique, Journal of The Electrochemical Society, 154(6) (2007) B577-B582. L. Zhang, H. Q. He, H. G. Wu, C. H. Li and S. P. Jiang, Synthesis and characterization of doped La9ASi6O26.5 (A=Ca, Sr, Ba) oxyapatite electrolyte by a water-based gel-casting route, International Journal of Hydrogen Energy 36 (2011) 6862-6874.)
Due to the high costs, the difficulty in recycling and the adverse environmental factors associated with many nano-sized catalysts there is a need for new forms of metal-oxide-based catalysts and methods for producing them.